Take something that large and as hard as a brick......slam it into ceramic tiling at that speed..........and you have damage, folks. Severe damage.
The foam was much lighter, and much more brittle, than a single brick that is about a tenth the size of the chunk that fell off, and it hit a glancing blow. I don't think it's a valid comparison.
No, the form section would have a relatively small delta V ( velocity difference). STS107 at that time was between mach 1.5 and mach 2 (@1500 mph) and the foam that separated is moving at the same speed. It has little mass to do any damge and would accelerate from drag in the slipstream between the tank and the shuttle. In less than 100 feet it may have slowed by 100 mph at point of impact with the shuttle. This would be the same as if the shuttle was on the ground at zero speed and the foam hit with a speed of @50-100mph.
From the photo of the foam break up after impact I would tend to believe the first point of impact was the leading edge and not underwing tiles. Grazing the tiles would not cause the falling tank foam to go to powder but hitting the leading edge first would. It is still unlikely this caused enough damge alone to be the single root cuase of failure but more study is needed.
You can only take into account the variance in velocities of the shuttle and foam - not the shuttle speed itself. We're more likely talking about a hundred MPH or so...
Also, the foam struck a glancing blow, so less energy would have been absorbed by the impact area.